1 V. Amplifying Devices and Microelectronics 1. Bipolar Transistors 2 Bipolar transistors in amplifiers 7 2. Field Effect Transistors 16 Junction Field Effect Transistors (JFETs) 16 Metal Oxide Semiconductor FETs (MOSFETs) 24 MOSFETs in amplifiers 34 3. Noise in Transistors 38 Noise in Field Effect Transistors 38 Optimization of Device Geometry 42 Noise in Bipolar Transistors 48 Noise Optimization – Capacitive Matching 56 Optimization for Low Power 61 4. SQUIDs 68 SQUID Noise 71 5. Microelectronics 74 Fabrication of Semiconductor Devices 74 Integrated Circuits 85 LBNL Microsystems Laboratory 89 Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 2 1. Bipolar Transistors Consider the npn structure shown below. COLLECTOR n - BASE + + p - IC - + + n - I - B EMITTER The base and emitter form a diode, which is forward biased so that a base current IB flows. The base current injects holes into the base-emitter junction. As in a simple diode, this gives rise to a corresponding electron current through the base-emitter junction. If the potential applied to the collector is sufficiently positive so that the electrons passing from the emitter to the base are driven towards the collector, an external current IC will flow in the collector circuit. The ratio of collector to base current is equal to the ratio of electron to hole currents traversing the base-emitter junction. In an ideal diode IID/ NLNDL C=nBE==nAnD np IBIpBEDp/ NDLpNADLpn Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 3 If the ratio of doping concentrations in the emitter and base regions ND /NA is sufficiently large, the collector current will be greater than the base current. Þ DC current gain Furthermore, we expect the collector current to saturate when the collector voltage becomes large enough to capture all of the minority carrier electrons injected into the base. Since the current inside the transistor comprises both electrons and holes, the device is called a bipolar transistor. Dimensions and doping levels of a modern high-frequency transistor (5 – 10 GHz bandwidth) 0 0.5 1.0 1.5 Distance [mm] (adapted from Sze) Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 4 High-speed bipolar transistors are implemented as vertical structures. (from Sze) The base width, typically 0.2 mm or less in modern high-speed transistors, is determined by the difference in diffusion depths of the emitter and base regions. The thin base geometry and high doping levels make the base-emitter junction sensitive to large reverse voltages. Typically, base-emitter breakdown voltages for high-frequency transistors are but a few volts. As shown in the preceding figure, the collector region is usually implemented as two regions: one with low doping (denoted “epitaxial layer in the figure) and the other closest to the collector contact with a high doping level. This structure improves the collector voltage breakdown characteristics. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 5 The result of this simple analysis implies that for a given device the DC current gain should be independent of current. In reality this is not the case. DC CURRENT GAIN vs. COLLECTOR CURRENT - 2N918 250 200 150 100 DC CURRENT GAIN 50 0 0.1 1.0 10.0 100.0 COLLECTOR CURRENT [mA] The decrease at low currents is due to recombination in the base- emitter depletion region. At high currents the current gain drops because of resistive voltage losses, shift of the high field region at the collector (increased base width), loss of injection efficiency as the carrier density approaches the doping concentration, etc. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 6 The “ideal” DC current gain depends only on device and material constants, whereas the recombination depends on the local density of injected electrons and holes with respect to the concentration of recombination centers. Thus, the relative degradation of DC current gain due to recombination depends on the current density. Within a given fabrication process, a large transistor will exhibit more recombination than a small transistor at the same current. Stated differently, for a given current, the large transistor will offer more recombination centers for the same number of carriers. As shown in the plots below, modern devices exhibit DC current gain that is quite uniform over orders of magnitude of emitter current. The figures also show the degradation of current gain after irradiation, here after exposure to the equivalent of 1014 minimum ionizing protons/cm2. The decrease of DC current gain at low current densities due to increased recombination is apparent. 100 50 NPN PNP 90 80 40 N N I I A 70 A G G P RE -RAD PRE -RA D T 60 T 30 N N E 50 E R R R 40 R 20 U PO ST RA D U C C 30 C C D 20 D 10 P OS T-RA D 10 0 0 -5 -4 -3 -2 -1 0 1 2 3 -5 -4 -3 -2 -1 0 1 2 3 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 EMITTER CURRENT DENSITY [mA/(mm)2 ] EMITTER CURRENT DENSITY [mA/(mm)2 ] In a radiation-damaged transistor the reduction in DC current gain for a given DC current will be less for smaller devices. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 7 Bipolar Transistors in Amplifiers Three different circuit configurations are possible: 1. Common Emitter Equivalent Circuit V+ LOAD LOAD SOURCE SOURCE 2. Common Base V+ SOURCELOAD SOURCELOAD V- 3. Common Collector (Emitter Follower) V+ SOURCE SOURCE LOAD LOAD Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 8 Although the bipolar transistor is a LOAD current driven device, it is often SOURCE convenient to consider its response to input voltage. Consider a transistor in the common emitter (CE) configuration. As discussed in a previous lecture, the voltage gain dVoutCdI AV===RLgRmL dVindVBE Since the dependence of base current on base-emitter voltage is given by the diode equation qeVBE//kBTqeVBEBkT IB=IRR(e-»1) Ie the resulting collector current is qeVBEB/ kT IIC==bbDCBDCRIe and the transconductance, i.e. the change in collector current vs. base-emitter voltage dICqqeeqeVBEB/kT gmº==b DCIRCeI dVBEkBBTkT The transconductance depends only on collector current, so for any bipolar transistor – regardless of its internal design – setting the collector current determines the transconductance. Since at room temperature kBT/qe= 26 mV I gI=»C 40 mC0.026 Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 9 The obtainable voltage gain of an amplifier depends on the output characteristics of the transistor. At low collector voltages the field in the collector-base region is not sufficient to transport all injected carriers to the collector without recombination. At higher voltages the output current increases gradually with voltage (saturation region), due to the change in effective base width. An interesting feature is that the extrapolated slopes in the saturation region intersect at the same voltage VA for Ic= 0 (“Early voltage”). The finite slope of the output curves is equivalent to a current generator with a shunt resistance C IC Ro E Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 10 where VA RKo = IC K is a device-specific constant of order 1, so usually it’s neglected. The total load resistance is the parallel combination of the external load resistance and the output resistance of the transistor. In the limit where the external load resistance is infinite, the load resistance is the output resistance of the amplifier. The maximum obtainable voltage gain is dICICVVAA Av,max =Ro=gRmo»= dVBEkBTq//eICkBeTq which at room temperature is about (40VA ). · Note that to first order the maximum obtainable voltage gain is independent of current. · Transistors with large Early voltages allow higher voltage gain. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 11 Bipolar transistors can be implemented as pnp or npn structures. The polarities of the applied voltages are the opposite: npn: positive collector-emitter and base emitter voltages pnp: negative collector-emitter and base emitter voltages The basic amplifier equations are the same for both transistor types. The availablility of complementary transistors offers great flexibility in circuit design. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 12 a) Common Emitter configuration Equivalent Circuit V+ LOAD LOAD SOURCE SOURCE The input signal is applied to the base, the output taken from the collector. The input resistance is proportional to the DC current gain and inversely proportional to the collector current. dVBEdVBEbbDCBkT DC Ri=»bDC == dIBdIgCmqIeC For bDC = 100 and Ic= 1 mA, Ri= 2600 W. Radiation Detectors and Signal Processing – V. Amplfying Devices and Microelectronics Helmuth Spieler Oct. 8 – Oct. 12, 2001; Univ. Heidelberg LBNL 13 Although the bipolar transistor is often treated as a voltage-driven device, the exponential dependence of base current on input voltage means that as an amplifier the response is very non-linear.